Overload protection for motors has been traditionally accomplished with the use of thermo-mechanical assemblies (heaters and bimetallic strips) which give a crude approximation of internal motor heating. The protection system monitors each of the phase currents that drive the motor and will trip or disconnect the motor from the line in a time which is inversely proportional to the square of the largest phase current, normally referred to as an I.sup.2 t characteristic. Although low in cost, the thermo-mechanical assembly does have drawbacks. As the ambient temperature varies, the current trip characteristics of the heaters and bimetallic strips also change. The I.sup.2 t trip characteristic of the system even at room temperature is only a rough model of motors heating due to an overload. Lastly, the technique does not lend itself very well to detecting motor heating due to phase imbalance or phase loss.
Motor protection designers have been looking for an alternative that is more accurate but competitive in cost with the thermo-mechanical assembly. Solid state electronics is an area of interest for the modular overload relay (MOR) designer because there is an ever-growing trend toward putting more calculative power in a smaller area for an ever-decreasing price. If a solid state MOR is used, then the method of sensing the current fed to the motor should change to take advantage of the "real time" monitoring capability that would be intrinsic in the MOR device. Presently, in three phase, three wire systems, current transformers, which are placed around each phase conductor in the three wire system, are used. This arrangement is immune to ambient temperature effects and phase loss detection is possible. However, the requirement of using three current transformers in additionn to the MOR device results in a more costly system than that of the thermo-mechanical assembly. Accordingly, it would be advantageous if the cost of the solid state protection system could be reduced. One means of accomplishing this would be to reduce the number of current sensors required. It is an object of the present invention to set forth the description of one approach that can be used for providing the phase current information while reducing the number of current sensors required to obtain this information.